This invention relates generally to the fabrication of semiconductor devices and circuits in which a conductive oxide layer is formed on a substrate surface and, more particularly, to a process for forming such an oxide without causing radiation damage or thermal damage to the substrate on which the oxide layer is formed.
2. Description of the Prior Art
In the fabrication of semiconductor devices and circuits, electrical contact to the active device regions is made through an electrode. The electrode material is characterized by being a good electrical conductor, and certain metals, such as gold, platinum, or aluminum, have been frequently used for this purpose. In addition, certain oxide materials which are electrically conducting have been used as an electrode member. In particular, certain conductive oxides which are also transparent to selected wavelengths of radiation have been useful for establishing electrical contact to the underlying substrate while at the same time allowing radiation to pass through the electrode to the substrate, such as in a front-side illuminated photovoltaic array. These transparent conductive materials are characterized by being both electrically conducting and transparent to selected wavelengths of radiation, as discussed, for example, by G. Haacke, in the publication entitled "Transparent Conducting Coatings," in Annual Review of Materials Science, Vol. 7, 1977, pages 73 to 93. In the past, thin films of silver, platinum, and gold were used to form transparent electrodes in photovoltaic and photoconductive cells. More recently, certain transparent conductive oxides, such as tin oxide and indium oxide, have been used for such purposes because of the high transparency, mechanical hardness, and environmental stability of these oxides. These transparent conductive oxides have been found to be especially useful for forming electrode members in semiconductor devices, such as charge-coupled devices, which, in turn, may be formed into a photodetector array. The conductive oxides used as electrodes in the past have been formed as a layer on the surface of a selected substrate, such as silicon, using known procedures, such as sputtering, thermal chemical vapor deposition, or evaporation, as described, for example, by J. L. Vossen in the publication entitled "Transparent Conducting Films", in Physics of Thin Films, Vol. 9, 1977, pages 1 to 71, and the references cited therein.
Using a known non-reactive sputtering process, a disk of the selected conductive oxide material, such as tin oxide (SnO.sub.2), is bombarded in a reaction chamber with inert ions, such as argon ions, which cause the SnO.sub.2 to vaporize from the target (disk); and the vaporized SnO.sub.2 subsequently deposits on the selected substrate. Using a known reactive sputtering process, a disk of the selected metal, such as tin, is bombarded in a reaction chamber with oxygen ions, which causes vaporization of the tin from the target, and the vaporized tin and oxygen ions then react to produce the desired oxide, which deposits on the substrate. In both the reactive and the non-reactive sputtering processes, the bombarding ions are formed by subjecting the chosen bombarding material, such as oxygen or argon, respectively, to a radio frequency (rf) or direct current (dc) discharge. However, as a result of the exposure of the chosen bombarding material to discharge, numerous extraneous ionized and neutral particles and high energy radiation with wavelengths as low as 500 angstroms or lower are produced. These extraneous particles then bombard the surface of the substrate on which the oxide is being formed and cause damage thereto by altering the quantity and distribution of charge therein. In addition, the bombardment of the substrate surface by these particles causes the formation of additional charged particles and radiation, which may also damage the substrate. This alteration in the charge of the substrate undesirably alters the electrical performance of the substrate and any structures formed therein. The damage produced by charge bombardment and radiation bombardment is particularly noticeable when the substrate comprises an electrically sensitive device, such as a charge coupled device or a device formed of certain compound semiconductor materials, such as mercury cadmium telluride, indium antimonide, or gallium arsenide.
Using a known thermal chemical vapor deposition (CVD) process for forming an oxide layer, a metal-organic compound, such as dibutyl tin acetate or indium acetylacetate, is thermally decomposed at the heated substrate surface to produce the desired tin oxide or indium oxide, respectively. However, such thermal CVD processes typically employ temperatures in excess of 500.degree. C., which are not compatible with certain temperature-sensitive compound semiconductor materials, such as mercury cadmium telluride, which undergo decomposition at elevated temperatures.
Using a known evaporation process to form an oxide layer, a source comprising the selected oxide, such as tin oxide, is placed in a reaction chamber and is raised to an elevated temperature sufficient to cause evaporation of the oxide, which subsequently deposits on the selected substrate. Alternatively, a known reactive evaporation process may be used in which a metal source is evaporated and the evaporant is reacted with oxygen at the substrate surface. However, the conductive oxide films formed by evaporation processes generally have poor surface morphology, high defect densities, and poor step coverage, all of which degrade the electrical properties of these oxide films.
The present invention is directed to the alleviation of the above-described prior art problem of imparting damage to sensitive devices due to charge bombardment and radiation bombardment or due to relatively high processing temperatures during the formation of a conductive oxide layer thereon